The present invention relates to a multilayer tube which withstands high temperatures and which has good impermeability qualities with regard to the transported fluid, and in particular motor vehicle fuel.
In the field of plastics fuel hoses, the mechanical and chemical quality requirements of the materials employed are ever increasing. The tubes must present practically zero permeability to the components of the fuel, while having the greatest possible mechanical stability at ever higher temperatures.
Research and development in this field has led to the production of multilayer tubes by coextrusion or successive extrusions of a plurality of plastics materials, where each layer is required to satisfy one of the performance criteria laid down. The qualities of each of the layers are aggregated.
In the current state of the art, there are many examples of combinations of materials comprising three or five successive layers in which one layer displays good impermeability to fuel, another serves as mechanical reinforcement over time and at high temperature, another provides the tube with effective protection against external attack, and yet others serve as adhesives between the different layers which are not necessarily naturally compatible, given the different natures of the tube materials being used (polyamides, polyethylenes, mixtures of two materials, polyesters, elastomers, thermoplastic elastomers, PTFE, PVDF, vinyl polychloride etc . . . ).
The majority of the solutions proposed in the prior art generally do not overcome other dimensional or economic constraints in satisfactory manner. Thus certain multilayer tubes possess the necessary qualities only when of a thickness that gives the tube a weight or a size that is difficult for a manufacturer to accept. Those or other solutions proposed mean that high-grade materials have to be used, but this leads to increased cost which is an economic break on their use.
FR 2 766 548 for example describes a two-layer tube with an inner layer of a polyphthalamide and an outer layer of an elastomer or a thermoplastic. One of the disadvantages of that tube lies in the rigidity of the inner layer and this damages the qualities of the connections at the ends.
The search for solutions must therefore take account of imperatives which go beyond mere technical performance and each tube is a particular compromise directed at optimizing the differing and sometimes contradictory requirements of regulations and of the market.
In this context, the invention provides a multilayer tube comprising at least an inner layer, an intermediate layer and an outer layer, the intermediate layer being made of a polyphthalamide (PPA). Polyphthalamide is a material possessing the quality of being dimensionally very stable even after a severe aging cycle. In addition sandwiching the polyphthalamide layer between two layers allows its thickness to be reduced without impairing its strength and its barrier function, thus making it possible to manufacture tubes that are flexible and easy to form.